In Chapter 3, the catalytic hydrogenation of alkenes and alkynes using (benzimidazolylmethyl)amine palladium(II) complexes is discussed. This work demonstrated the influence of the coordinating and non-coordinating solvents in the catalytic hydrogenation of alkenes.
Introduction
Introduction and literature review of transition metal complexes as catalysts in hydrogenation and methoxycarbonylation reactions. The catalytic hydrogenation and methoxycarbonylation of unsaturated hydrocarbons such as alkenes and alkynes are some of the important reactions in which various transition metal complexes have been employed.
Catalytic methoxycarbonylation of alkenes and alkynes
Mechanism of palladium (II) catalysed methoxycarbonylation reactions
The Pd-H complex forms Pd-alkyl and Pd-acyl intermediates in the presence of ethane and carbon monoxide. A different approach was used by Cavinato et al.9, synthesizing trans-[Pd(COOCH3)(PPh3)2(TsO)], an intermediate formed in the carbomethoxy mechanism.
Hydrogenation reactions of alkynes and alkenes
Heterogeneous catalytic hydrogenation of alkenes and alkynes
Homogeneous catalytic hydrogenation of alkenes and alkynes
Application of transition metal catalysts in catalytic homogeneous
Rhodium and ruthenium-based catalysts in hydrogenation reactions of
- Rhodium-based catalysts in the hydrogenation of alkenes
- Ruthenium-based catalyst in the hydrogenation reactions
Ruthenium(II) complexes have been successfully used as catalysts in the hydrogenation of a range of alkenes and alkynes. However, these compounds are very sensitive to air and easily poisoned.53 Other ruthenium complexes of the type [HRu(CO)Cl(L)2] (L = PPri3.. PBut2Me) have been reported in the hydrogenation of alkenes and alkynes. and are very efficient catalysts.54,55.
Palladium-based catalysts in catalytic hydrogenation
- Phosphine-donor palladium complexes
- Nitrogen-donor palladium complexes
- Hemilabile palladium(II) complexes as hydrogenation
Kinetic studies of the hydrogenation of phenylacetylene provided clues that could help elucidate the catalytic cycle of the reaction.40. Mechanism of the hydrogenation of alkynes with palladium(II) complexes. In the absence of a catalyst, the rate of thermodynamically favored the hydrogenation.
Mechanism of the hydrogenation of alkynes with palladium(II) complexes….22
But this species (B) readily collapsed to C via low-barrier processes according to level DFT calculations, using an extended basis set.77 Attempts were made to detect the palladium hydride species (B or C), but these were not successfully detected using PHIP NMR in the hydrogenation of 1-hexyne. As such, at the end of a reaction, the catalyst remains in one phase and the product is in the other phase.81 These two-phase systems allow the catalyst to be recycled without losing its superior homogeneous catalytic activity and selectivity, and makes it also facilitates separation of catalysts.81 To date, numerous water-soluble transition metal-based catalysts have been used in biphasic hydrogenation reactions.80 Examples of water-soluble complexes used in biphasic hydrogenation reactions include cationic 1'1-bisquinoine-ruthenium complexes (Fig. 1.11)82 , iridium, rhodium and ruthenium complexes (Fig. 1.12)80.
Rationale of study
Objectives
General objectives
Specific objectives
29 v) Performing theoretical calculations (DFT) that will provide important additional information and explain the experimental results. The results of the work carried out in attempts to achieve the above objectives are described in Chapters 2 to 6, while Chapter 7 summarizes the main findings and conclusions from the entire experiments. The first author's contributions include: syntheses and characterization of the compounds, methoxycarbonylation reactions, and preparation of the manuscript.
Introduction
Such examples include palladium complexes of the type [PdCl2(Ph2PNHpy-k2-P,N)] and [PdCl(Ph2PNHpy-k2-P,N)(PPh3)]Cl, which provide active and stable catalysts in the methoxycarbonylation of styrene.5 . In addition, internal olefin substrates have been used to investigate the effect of the position of the double bond on the regioselectivity of the ester products. In addition, studies have been carried out on the nature of active species, which will be discussed.
Experimental section
Materials and methods
Synthesis of palladium(II) complexes
General procedure for the methoxycarbonylation reactions
At the end of the reaction time, the reaction was cooled, excess CO was vented, and samples were taken for GC analysis to determine percent conversion of alkene substrate to ester. The identities of the ester products were determined using authentic standard samples and mass spectral data.
Results and discussion
Synthesis and characterization of the palladium complexes 1-
For example, the 1H NMR spectra of L1 and its corresponding complex 1 showed CH2 proton signals at 4.65 ppm and 5.33 ppm, respectively. In addition, the CH3 proton signal at 2.32 ppm for 1 confirmed the coordination of p-TsO- anion to the palladium atom (Figure 2.1). 44 Figure 2.4: ESI-MS of the complex [Pd(OTs)( PPh3)(L1)] (1) with the inset showing mass spectrum of the calculated and found isotopic distribution.
Methoxycarbonylation reactions using palladium complexes 1-5 as
- Effect of catalyst structure and phosphine derivatives
- Investigation of the effects of solvent and acid promoter on
- Methoxycarbonylation of internal olefins using catalysts 1 and 4
The regioselectivity of the ester products was also influenced by the nature of the auxiliary phosphine ligands (Table 2.1, entries 4-10). We then studied the effect of the solvent system and the nature of the acid promoter on the catalytic activity of complex 1 using the substrate 1-hexene (Table 2.2, entries 1-5). 56 Figure 2.8: GC spectrum (A) and MS spectrum (B) of the branched product obtained by metacarbonylation of trans-2-octene using catalyst 1.
Conclusions
Contributions of the first author include: performing the catalytic reactions and drafting the manuscript.
Introduction
For example, Bacchi et al. and Drago et al. used hydazonic phosphine palladium(II) and bidentate (2,5-dimethylphospholano) palladium(II) complexes as effective catalysts in the hydrogenation of alkenes. Although the phosphine donor palladium(II) catalysts have been successfully used in the homogeneous hydrogenation reactions of alkenes and alkynes, these systems suffer from a lack of stability and sensitivity to moisture and air.11 As a result, nitrogen donor palladium (II)) catalysts are emerging as suitable alternatives due to their better stability and ease of synthesis compared to the phosphine-donor palladium(II) complexes. For example, the pyridine-2-carbaldinepalladium(0)12 and bis(arylimino)acenaphthenepalladium(0)13 complexes have been shown to exhibit good selectivity and stability in the homogeneous hydrogenation of alkynes.
Experimental section
Materials and methods
Density Functional Theory (DFT) studies
General procedure for hydrogenation reactions of alkenes and alkynes The catalytic hydrogenation reactions were carried out in a stainless steel autoclave.
General procedure for the hydrogenation reactions of alkenes and alkynes….67
The solution mixture was flushed three times with hydrogen before the autoclave was finally filled with hydrogen, and the pressure and temperature were adjusted to 5 bar and 30 ¯C, respectively. The stirring speed was set at 500 rpm and stirring started when the temperature reached equilibrium. The mixture was stirred under constant hydrogen and the sample was withdrawn after 1.5 h, filtered using 0.45 µm microfilters and analyzed by Varian CP-3800 GC (ZB-5HT column 30 m × 0.25 mm × 0.10 µm) GC instrument to determine the percentage conversion of styrene to ethylbenzene.
Results and discussion
Hydrogenation reactions of alkenes and alkynes using palladium(II) complexes
In order to fully account for the role of complexes 6-11 in the observed catalytic hydrogenation reactions, control experiments were performed without the use of palladium(II) complexes and also in the presence of ligand L2 only under similar reaction conditions. 70 in 10 h (Table 3.1, entries 7 and 8) confirmed that complexes 6-11 were responsible for the observed catalytic activities. Thus, we further carried out kinetics, selectivity, theoretical and mechanistic studies of hydrogenation reactions of alkenes and alkynes using complexes 6-11 as catalysts.
Kinetic studies of styrene hydrogenation
- Effect of complex structure on catalytic hydrogenation of styrene by 6-
- Effect of temperature and solvents on styrene hydrogenation
- Effect of the nature of the alkene and alkyne substrates on styrene
Kinetic experiments were further performed to determine the effects of catalyst concentrations on the hydrogenation reactions of styrene using complexes 7 and 11. The rate law for hydrogenation reactions of styrene using catalyst 7 can therefore be expressed as given in Equation 4.4. 77 Table 3.2: Effect of catalyst concentration and pressure on the hydrogenation of styrene using catalyst 7 and 11a.
![Figure 3.5: Plot of In(k obs ) vs In[7] (a) and In[11] (b) for the determination of the order of reaction with respect to catalyst 7 and 11 in the hydrogenation of styrene](https://thumb-ap.123doks.com/thumbv2/pubpdfnet/10643815.0/111.892.118.842.578.865/figure-plot-determination-reaction-respect-catalyst-hydrogenation-styrene.webp)
Theoretical insights of the hydrogenation reactions of alkenes
85 Table 3.5: DFT calculated data for palladium (II) complexes using the B3LYP/LANL2DZ level of theory. 86 Table 3.6: DFT-calculated HOMO and LUMO frontier molecular orbitals of palladium(II) complexes 7-10 using LANL2DZ for palladium and 6-311G for all other atoms. 87 Figure 3.12: Plot of TOF vs NBO charges for palladium(II) metal atom showing a linear correlation between catalytic activities of complexes 6-10 and palladium(II) atom NBO charges.
Proposed mechanism of the hydrogenation of styrene
Coordination of the styrene substrate to 7a provides the Pd-styrene adduct (7b) as deduced from the base peak at m/z = 514 amu. A Markovnikov migration of the hydride to the coordinated substrate to form a 14-electron Pd-alkyl species (7c) was established from the m/z signal at 510 amu. 90 Attempts to confirm the formation of the Pd hydride intermediates using 1H NMR spectroscopy were not successful, even at elevated temperatures (Figure 3.14).
Conclusions
We did not observe any signal related to Pd-H (around -5 ppm), which may be due to the low pressures (atmospheric) used in the NMR studies, as opposed to the higher pressures of 5-30 bar used in the catalytic experiments. The electrophilicity of the metal palladium atom of the complexes, as supported by DFT calculations, increased the reactivity of the corresponding catalysts. A mechanistic pathway involving the formation of a palladium monohydride intermediate as the active species was established by mass spectrometry.
The contributions of the first author include: synthesis, characterization of the compounds, performance of the catalytic reactions and drafting of the manuscript.
Introduction
Hydrogenation of alkenes and alkynes catalyzed by N^O (imino)phenol-palladium(II) complexes: structural, kinetic and chemoselectivity studies. 95 Another approach currently gaining momentum in the design of active and stable catalyst systems is the hemilabile ligands, first introduced by Jeffrey and Rauchfuss.14 The hemilabile nature of the ligands in these catalysts has been demonstrated in a number of reports of palladium( II) catalysts with P^N^O15, P^N^S16, N^N^S17; S^O^S donor sites.18 In one such study by Bacchi et al.16, palladium(II) complexes of hemilabile potential P^N^S ligands were found to be inactive due to the strong coordination ability of the pendant S donor atom, limiting substrate coordination, that is, the ligand adopts tridentate coordination upon activation. The higher catalytic activities of S^O^S and S^O^O systems are attributed to the weak coordinating ability of the O donor atom compared to the S atom.18.
Experimental section
- Material, instrumentation and methods
- Synthesis of (ethylimino)ethyl)phenol amine ligands and palladium(II)
- General procedure for the hydrogenation reactions of alkenes and
- General procedure for kinetics exepriments…
- Density Functional Theoretical (DFT) studies
At the end of the reaction time, the reaction was cooled, excess hydrogen was blown off and the samples were drawn, filtered with 0.45 µm microfilters and analyzed by gas chromatography to determine the percentage conversion of styrene to ethylbenzene. Standard authentic samples were used to confirm the presence and composition of the hydrogenation products. Kinetics of the hydrogenation reactions were investigated for complexes 12-17 by monitoring the reactions using Gas Chromatography.
Results and discussion
Synthesis and characterization of (ethylimino)ethyl)phenol amine ligands and
Hydrogenation reactions of alkenes and alkynes catalysed by complexes
- Preliminary screening of palladium complexes 12-17 in molecular
- Influence of complex structure on the kinetics of hydrogenation
It is clear that the catalytic activity of the complexes was influenced by the hanging arm of the ligands. 117 Figure 4.8: Graph of In[Sty]0/[Sty]t versus time to determine the dependence of the reaction rates on the catalyst concentrations using catalyst 12. 121 Figure 4.10: Graph of In[Sty]0/[Sty ] ]t versus time to determine the dependence of reaction rates on dihydrogen pressure using catalyst 12.
Substrate scope and chemo-selectivity studies using complex
In terms of production distribution, hydrogenation of alkenes was accompanied by isomerization reactions, while alkyne hydrogenation reactions occurred in two steps to sequentially produce the respective alkenes and alkanes (Fig. 4.14). To gain more insight into the isomerization reactions and incomplete conversions of internal alkenes to alkanes, we used internal alkenes trans-2-hexene and trans-2-octene substrates (Table 4.4, entries 7 and 8). Thus, a significant decrease in catalytic activity observed from 72% to 44% observed for 1-octene and trans-2-octene, respectively, can explain the incomplete hydrogenations of the internal alkenes to their corresponding alkanes (Fig. 4.14a).
Evidence of hemi-lability from DFT studies
Conclusions
155 Figure 5.8: Plot of In(kobs) vs In[18] to determine the order of reaction with respect to catalyst 18 in the hydrogenation of styrene. 158 Figure 5.10: Plot of kobs vs PH2 (bar) to determine the reaction order with respect to H2 pressure in the hydrogenation of styrene using catalyst 18. It was observed that the catalytic activities of 18 depended on the nature of the substrate.
Introduction
A number of palladium(II) complexes have been successfully used in the homogeneous hydrogenation of alkenes and alkynes. Established examples include palladium(II) complexes derived from bidentate phosphines1 of the P^N2 type and nitrogen donor3 catalysts of the N^S4 and N^O.5 Although the bidentate nitrogen (N^S, N^O) and Because phosphine donor (P^N)-palladium(II) catalysts have been successfully used in the homogeneous hydrogenation reactions of alkenes and alkynes, these systems suffer from a lack of stability.6 For example, Ogweno et. Therefore, in this chapter the synthesis and characterization of palladium(II) complexes of hybrid N^P donor ligands and their applications in the catalytic hydrogenation of alkenes and alkynes are reported.
Experimental section
Material, instrumentation and methods
The change from hard-hard donor atoms (N^O) to hard-soft donor atoms (N^P) is because it has been reported in the literature that good results in terms of activity, selectivity, robustness and efficiency of the catalyst can be obtained with complexes containing both a 'soft' P and 'hard' N donor atom.9 For example, excellent turnover rates of up to 120,000 h-1 in the transfer hydrogenation of acetophenone were reported by the groups of Baratta and Hermann, using ruthenium(II) complexes anchored on N^P ligands.10 However, a lower turnover frequency (333 h-1) was reported for N^O ruthenium(II) complexes.10.
Synthesis of P^N (diphenylphosphino)benzalidene ethanamine ligands and
At the end of the reaction time, the reactor was cooled and the excess hydrogen was removed. By comparing the peak areas of styrene and ethylbenzene at regular intervals, the conversion rate of styrene to ethylbenzene could be determined. Kinetics and the influence of the complex structure on the molecular hydrogenation of styrene were investigated for complexes 18-23 by monitoring the reactions GC chromatography.
